Personal digital assistants (PDAs), cell phones, digital cameras, laptop computers and other portable electronic devices often use a type of infrared transceiver called an IrDA (Infrared Data Association) transceiver to transfer information. FIG. 1 illustrates a typical IrDA transceiver 1. Transceiver 1 is a multi-chip module that is enclosed in a metal shield 2. FIG. 2 is a cross-sectional view of the IrDA transceiver. FIG. 3 is a top-down cross-sectional view of the circuitry within the IrDA module. The IrDA module includes three dice: 1) an IR transmitter diode die 3, 2) an IR receiver PIN diode die 4, and 3) a controller die 5. The three dice are mounted on a very small circuit board 6. Molded plastic 7 covers the printed circuit board and die assembly and forms two semi-spherical lenses 8 and 9. Lens 8 is to focus incoming IR radiation 10 onto the PIN receiver diode 4. Lens 9 is to redirect the radiation from the IR transmitter diode die 3 into a beam 11.
Electronic consumer devices in the home such as televisions, VCRs, DVD players, stereo equipment, home theatre equipment, CD players and so forth are typically controlled by remote control devices that transmit IR signals. Although the various manufacturers of electronic consumer devices use different coding and modulation schemes to control their electronic consumer devices, the overall use of IR to control such electronic consumer devices is referred to here as “RC”.
Although both IrDA and RC both use IR to communicate, the two use different wavelengths of IR. IrDA typically uses an infrared signal in the range of 850 nm to 900 nm (for example, 875 nm). RC remote control devices typically use an infrared signal of 900 nm to 950 nm (for example, 940 nm). In addition to the IR wavelength, IrDA and RC use very different protocols. For this reason, IrDA and RC have been considered two different communication schemes.
It has been recognized, however, that the IrDA transceivers so common in PDAs, cell phones, digital cameras, and laptop computers all include IR transmitter diodes. It would be advantageous if these IrDA IR transmitters could be used to control electronic consumer devices in the home. One PDA manufacturer has endeavored to have the IrDA module in its PDAs be capable of controlling electronic consumer devices in the home. PDAs are currently on the market with this functionality.
FIGS. 1-4 illustrate the design employed by this PDA manufacturer. For additional information, see: 1) “Agilent HSDL-3003 IrDA Data Compliant Low Power 115.2 kbit/s with Remote Control Infrared Transceiver—Data Sheet,” 21 pages, June 2003, 2) “Interfacing the Agilent HSDL-3002 for Remote Control Operation—Application Note 1314,” 16 pages, February 2003, and 3) “Agilent IrDA Data Link Design Guide,” 67 pages, March 2003.
Because RC electronic consumer devices typically respond to IR signals at 940 nm, an 940 nm RC transmitter diode 12 is included along with the 875 nm IrDA transmitter diode 3. When an IRDA transmission is to be made, then the IrDA driver portion of controller die 5 pulls current through the 875 nm IrDA transmitter diode 3 in standard IrDA fashion. When an RC transmission is to be made, then an external FET 13 is made conductive to pull current through the added 940 nm RC transmitter diode 12.
In order to be able to provide the coding information needed to control all the many electronic consumer devices on the market, the PDA manufacturer has consulted a manufacturer of IR remote controls. The remote control manufacturer evidently will not provide the PDA manufacturer the coding information independently of hardware. The remote control manufacturer is evidently requiring the PDA manufacturer to buy microcontrollers from the remote control manufacturer that have the coding information programmed into them. The PDA manufacturer is therefore forced to provide a microcontroller 14 (that includes the coding information) in each PDA it manufacturers.
Although it might seem that the functionality of FET 13 could be included on controller die 5, the PDA manufacturer has evidently chosen not to do this. The reason may be that FET 13, in order to pull the required amount of current through the 940 nm diode, has to have a low on-resistance. Realizing a FET with such a low on-resistance would take up so much integrated circuit area that controller die 5 would be so large that it could not any longer fit into the confining space of the standard IrDA form factor. Providing the two transmitter diodes into the form factor is difficult even without increasing the die size of controller die 5. Accordingly, the design employed on the PDAs involves at least three components external to the IrDA module: 1) the FET 13, 2) the microcontroller 14, and 3) a current-limiting resistor 15. FIG. 1 illustrates the external FET component 13 as well as external resistor 15. External microcontroller 14 is not pictured.
Not only does the PDA design set forth above involve an external microcontroller, a FET and a resistor, but the design also requires an extra IR transmitter LED 12 to be provided in the IrDA module. Providing this IR LED die adds cost to the system. In addition, IrDA module assembly can be more expensive because the placement of the two IR LEDs is somewhat difficult. The two IR LED diode dice need to be placed close to one another in order to be satisfactorily aligned with respect to the optics of lens 9. Very importantly, the PDA manufacturer is forced to buy and power a microcontroller just to get the coding information needed to interface to RC electronic consumer devices.
It has been recognized that an 886 nm IrDA transmitter LED within an IrDA module can be used as a transmitter both for IrDA and RC applications. Although the peak wavelength of the 886 nm IrDA transmitter LED and the wavelength of peak sensitivity of a remote control receiver are not matched, radiation transmitted by the 886 nm transmitter LED is nonetheless received at the remote control receiver. Due to the mismatch, however, it may be necessary for the IrDA transmitter LED power to be increased in order for it to work properly in a remote control application. A recent application note from Vishay Semiconductors (Vishay Semiconductor Application Note entitled “Utilizing a Vishay IrDA Transceiver for Remote Control”, document number 82606, 14 pages, Feb. 20, 2004) discloses reducing the value of a current-limiting resistor so as to increase IrDA LED peak current and thereby to increase emission intensity of the IrDA LED transmitter. Laboratory test results are described, but a workable and cost-effective mechanism or circuit for reducing the value of the current-limiting resistor is not set forth.